In the military the rack is your bunk. To rack out or hit the rack is to go to bed for the purpose of sleeping. If you find it difficult to get out of bed through fatigue, laziness, illness, hangover, depression, or any of a number of other reasons, then the "rack Monster" has ahold of you.

"I know we were all going to see Tina the Cat Woman today, but the Rack Monster had ahold of me and wouldn't let go."